Direct Estimation of Retained Quantity

DOE-STD-1121-98

7.3.1 Direct Estimation of Retained Quantity

When thorough bioassay histories are attainable, and good confidence can be placed on organ and

whole body radionuclide content evaluations, it is possible to explicitly derive the retained quantity and

retention history of an exposure without resorting to use of default parameters. In some cases the

uncertainties associated with the biokinetics are much greater than the uncertainty in the direct

assessments of intake and retained quantity.

A tritium exposure with sufficient urine assay data to document the biological excretion rate is an

example of using excretion history. Due to uncertainty in the route of intake (e.g., skin absorption versus

inhalation) and in the biological clearance rate (which depends on water consumption), the tritium

excretion history provides the best assessment of the number of transitions and, thus, the dose equivalent.

Similarly, a radioiodine exposure, well documented in time and monitored by in vivo thyroid counting,

can be assessed directly from the bioassay result. In both cases, discrete or parameterized methods of

summation of transitions in the well-known source organs will provide sufficient information for dose

assessment.

Where direct uptake and retention history are used for dose assessment, the method for converting

data to dose equivalent should be documented as part of the dose assessment. However, if the bioassay

data are insufficient for a thorough assessment of retained quantities, or are of such poor quality that

whole body or pertinent organ content cannot be directly derived, then biokinetic models should be used.

7.3.2 Biokinetic Modeling

A biokinetic model is a time-dependent mathematical representation of the relationship between

intake, uptake, retention, translocation, and excretion for radionuclides taken into the body. Models differ

in their scientific approach and mathematical formalism. Some models, such as systemic uptake excretion

models, are empirically derived from studies of radionuclide behavior in humans or animals. Other

models are derived from considerations of the fundamental physiological and biochemical processes of

the body.

The application of biokinetic models for internal dosimetry has been described by the NCRP (NCRP

1985a, 1985b), the ICRP (ICRP 1968, 1969, 1973, 1979a, 1979b, 1980a, 1980b, 1981a, 1981b, 1982a,

1982b, 1982c, 1986b, 1988, 1989b, 1993b, 1994a, 1994b, 1995), and many distinguished authors

(Avadhanula et al. 1985. Cabello and Ferreri 1993; Calvo and McLaughlin 1995; Carbaugh et al. 1989;

Chang and Snipes 1991; Fauth et al. 1996; French et al. 1996; Hill and Strom 1993; Inkret and Miller

1995; Johnson and Carver 1981; Johnson and Myers 1981; Lawrence 1978; Lessard et al. 1987; Skrable

et al. 1994b; Sula et al. 1991).

7.3.2.1 Selection of Biokinetic Models

Biokinetic models at a facility should be documented and used consistently. If an exception to the

documented model is appropriate, the alternative method should be justified and documented in the dose

assessment. Normally, models developed or endorsed by the DOE, ICRP, NCRP, or ANSI should be

used. Limitations of these models should be recognized, and the models should be used for their intended

purpose. (An example may be the use of biokinetic models in the ICRP Publication 30 series that

describe the retention of radionuclides in the body. The ICRP models generally employ linear first-order

kinetics to simplify the mathematical representation, ignoring recirculation between organs and the

systemic compartment. Models that have been developed from empirical excretion functions or those that